Lasers are presently employed for a wide variety of applications. For example, lasers are employed to process materials, such as by cutting, welding, heat treating, drilling, trimming and coating materials, stripping paint, removing coatings, cleaning surfaces, and providing laser markings. Lasers are also used in many medical applications for precision surgery. Additionally, lasers are used in military applications, including laser weapon and laser ranging systems. Laser communication systems have also been developed in which laser signals are transmitted in a predetermined format to transmit data.
Along with the ever increasing number of applications in which lasers are used, the demands on the laser systems are also ever increasing. For example, a number of applications, including military, materials processing, medical, and communications applications, demand continuous wave lasers which emit increasingly higher power levels. In addition, a number of applications demand that the laser system produce an output beam which is of high quality, e.g., exhibiting predominantly or entirely fundamental or TEM.sub.00 mode characteristics. Accordingly, the output beam can be more definitely focused to achieve higher brightness. At the same time, many applications require that the laser system produce an output beam which is adaptable or dynamically controllable.
One example of the need for high power, high quality laser beams is illustrated in laser devices used for focusing on remote targets. In these applications, it advantageous for the laser beam to achieve a maximum brightness at the location of the target. For example, in military applications, it is advantageous to generate a laser beam that is focused on the remote target with maximum intensity. Similarly, in medical applications, it is essential that the laser beam is focused at the target tissue such that surrounding tissue is not affected. However, an overall problem with the control of laser beams is perturbations in the atmosphere in which the laser beam propagates. These perturbations degrade the laser beam, deflect the laser beam, and reduce laser power.
To address the problems associated with these perturbations and provide control of the laser beams, devices have been developed that sense the perturbations occurring in the path of the laser beam and compensate for these perturbations by adjusting the laser beam. For example, The Boeing Company, assignee of the present application, has developed several different types of laser devices that generate high powered, turbulence compensated laser beams. Examples of these device are discussed in detail in U.S. Pat. No. 5,694,408 to Bott et al., U.S. Pat. No. 5,847,816 to Zediker et al., and U.S. Pat. No. 5,832,006 to Rice et al., the contents of which are incorporated herein by reference.
The basic approach to many of these devices is to amplify a coherent signal emitted from a master oscillator using a phased array of fiber optic amplifiers. A portion of the output optical signal referred to as a power signal is extracted for comparison to a reference laser beam also output by the master oscillator. The power signal and the reference signal are combined by interference, and the interference signal is sampled by an array of detectors. The difference in phase between the power and reference signal is recorded by the detector, and is used as feedback for altering the phase modulation of the power signal via an array of phase modulators that are in optical communication with respective fiber optic amplifiers.
As an example, in one application, a reference beam is initially transmitted to a target of interest, and the reflection of the beam indicates atmospheric turbulence in the path of the output laser beam. To counteract these turbulence, the device alters the phase of signals emitted by the various fiber optic amplifiers such that the output laser has a wavefront that compensates for the atmospheric turbulence. An important component of this device is the feedback loop used to control the phase modulation of the output laser beam. Specifically, as discussed, a portion of the output laser beam is combined through interference with a reference signal to determine the phase difference for the signals emitted by each fiber optic amplifier. By use of the feedback signal representative of the phase of the output laser beam and knowledge of the desired wavefront, the output laser beam can be controlled via the array of phase modulators to produce the desired wavefront and/or to appropriately steer or tilt the wavefront.
An important aspect of these laser devices is the control of the phase of the output laser beam by the array of phase modulators. As discussed, these phase modulators are controlled by a feedback signal representing the difference in phase of a portion of the output laser beam called the power signal and the reference signal. Although these systems, for the most part, provide reliable and accurate control of the output laser beam, problems may be encountered when the feedback signal exceeds a desired maximum feedback value such that saturation may occur. An additional problem is experienced when the feedback signal causes uncontrolled modulation changes in the output signal.
The problems associated with the phase modulators are illustrated with reference to FIG. 1. FIG. 1 is a block diagram representation of a typical Mach-Zehnder interferometer that uses a feedback signal to correct for phase differences. Specifically, the laser device 10 includes master oscillator 12 that produces an output optical signal 14. This output optical signal is directed to a beam splitter 16 that creates a power and a reference signal 18. The reference and power signals are amplified by respective fiber amplifiers 20 and 22 and pump sources 24 and 26. The reference and power signals are collimated by collimating lenses 28 and 30. A portion of the power signal is separated by a beam splitter 32, and the reference signal and the power signal are combined by interference to produce an interference signal 34. The interference signal is detected by a detector 36 and supplied to a lockin amplifier 38.
Prior to interference with the output signal, the reference signal is modulated with a reference phase modulator 40 at a predetermined frequency provided by a frequency generator 42. The predetermined frequency is also supplied to the lockin amplifier 38, which generates a lockin signal that is proportional to the sine of the phase difference between the reference and power signals at the predetermined frequency. This lockin signal is provided to an optical phase modulator 44, which alters the phase of the power signal to substantially match the phase of the reference signal. As such, the phase of the output signal may be regulated. The feedback loop may also include a gain amplifier 46 that amplifies the lockin signal prior to input into the phase modulator.
As discussed above, problems may be encountered where the lockin signal either exceeds the maximum input of the phase modulator or the lockin signal is such that it may introduce uncontrollable phase changes in the output signal. This problem is more specifically illustrated with reference to FIGS. 2 and 3. FIG. 2 illustrates in block diagram form the feedback loop of the laser device of FIG. 1. Specifically, the interference signal received by the detector represents phase difference .DELTA..phi..sub.tot between the reference and output signals and consists of gain differences and environmental phase differences .DELTA..phi..sub.Env. The interference signal 34, shown in FIG. 1, is provided to the lockin amplifier, which, in turn, generates a lockin signal that is proportional to the sine of the phase difference between the reference and output signals (i.e., sin(.DELTA..phi..sub.tot)). The lockin signal is also gain amplified by the gain amplifier 46 (i.e., gsin(.DELTA..phi..sub.tot)), prior to being presented to the phase modulator, which alters, i.e., reduces, the phase of the signal by a corresponding amount. As such, the relationship between .DELTA..phi..sub.tot and .DELTA..phi..sub.Env is expressed as follows: EQU .DELTA..phi..sub.tot =.DELTA..phi..sub.Env -gsin(.DELTA..phi..sub.tot)
As illustrated by the equation, a shift in environmental phase between the reference and power signal will directly affect the value of the feedback signal. With reference to FIG. 3, the drift in phase between the reference and output signals of a typical optic interferometer is illustrated. As can be seen, the interferometer may experience large phase drifts due to environmental causes over a short period of time. These large phase drifts may cause problems with operation of the phase modulators. Specifically, typical phase modulators have maximum input limits, above which, the phase modulators will saturate. In systems such as described above, if the environmental changes generate a signal that is greater than a desired maximum lockin signal, saturation may occur, which will affect operation of the laser device.
An additional concern with the feedback signal is related to sinusoidal aspects of the feedback signal. With reference to FIG. 4, the feedback signal for a particular gain value of g=10 is plotted in terms of .DELTA..phi..sub.tot vs. .DELTA..phi..sub.Env. As can be seen from this graphic representation, for certain environmental phase difference values, the change in total phase may be such that modulation of the power signal is uncontrollable. For example, if the environmental phase difference is currently .DELTA..phi..sub.Env /2.pi..apprxeq.0.56, then the total phase difference is .DELTA..phi..sub.tot /2.pi..apprxeq.-0.8. However, if in the next instant, the environmental phase changes to .DELTA..phi..sub.Env /2.pi..apprxeq.0.77, the total phase difference will change to .DELTA..phi..sub.tot /2.pi..apprxeq.-0.5. This change in phase may cause uncontrollable phase changes in the output signal.